This invention pertains generally to continuous wave (CW) radar systems and particularly to an improved signal processing technique for processing a frequency modulated (FM) CW waveform to obtain unambiguous target range and range gate (Doppler) in a single radar dwell.
It is known in the art that a CW radar may be modified so that the range of a selected target may be measured. Thus, if the transmitted signal periodically is frequency modulated (by a linear sawtooth, triangular or sinusoidal modulation signal) in an appropriate manner, the range of any target may be determined by correlating all echo signals with differently delayed replicas of the transmitted signals to find the propagation delay of the echo signals from each target. Range then is determined where the correlation process indicates the best match between a particular delayed replica and received signal. One example of a known CW radar adapted to range measurement is shown and described in U.S. Pat. No. 4,176,351 assigned to the same assignee as the present application. In the just-cited patent the modulation signal is sinusoidal (262 Hz with a peak carrier deviation of 3 KHz), with eight tapped quartz delay lines and a corresponding number of correlation mixers used to effect the requisite correlation process. Range rate of each target, as indicated by the Doppler shift frequency of the echo signals from each target, is then determined in a conventional way from signals received during periods in which the transmitted signal is unmodulated CW. Although the CW radar shown in U.S. Pat. No. 4,176,351 is effective in many applications, the quartz delay lines limit the number of applications. For example, it is difficult, if not impractical, to use a CW radar as taught in U.S. Pat. No. 4,176,351 in guided missiles. The size, weight and susceptibility of quartz delay lines to damage from shock, vibration and temperature extremes are the causes of difficulty in such applications. Further, in order to reject signals from stationary clutter, the CW radar contemplated in U.S. Pat. No. 4,176,351 is arranged to downconvert received signals to a first intermediate frequency, then to filter out returns from stationary clutter and then to upconvert any remaining signals to a second intermediate frequency by heterodyning any such remaining signals with a local oscillator signal having the same deviation as the transmitted signal. It is, however, difficult to accomplish downconversion and upconversion (with a desired redeviation of the finally obtained signals) without introducing undesired intermodulation effects. Further, because an excessively large number of range/Doppler sidelobes are engendered by a sinusoidally modulated FM waveform, targets with the same Doppler shift frequency at different ranges as well as targets with small cross-sections in the presence of targets with large cross-sections are difficult to detect.